I've always said it's how you're brought up. If you're brought up watching a particular sport you'll probably be watching it for the rest of your lives. I'd say that's the case for 95% of football fans all over the world. I mean look at how boring baseball is, but because so many Americans are brought up watching it - they love it. :p

America already has 3 main national sports and a 4th in Hockey if the NHL ever gets sorted out. Football will always be a minority sport there.